Vst Instrument goes silent in Export Audio

Hi can anyone help? I’m using Cubase 5 LE. I created a virtual instrument drum track in midi then applied the Halion VST. The rest of the song is imported wave files on the remaining tracks. It’s about 7 mins long. And there are 11 wav recordings and the instrument track. When I export all to a wav mixdown all seems OK. But when I play the wav file, the drums go silent half way through. All the other audio recordings remain. If I mute the wav recordings and export again, I can hear the whole drum track. Then If I mute the drum track and export the 11 wav recordings I can hear them all. Why should my virtual instrument drum track go quiet? I,m using Win 7 64 bit Home premium OS.

Maybe something is sending a message to mute the vst. The Halion vst works OK, it seems to be third parties that used to work but don’'t now.

Maybe freeze the offending track first, then export.